The present invention pertains generally to a dual-cure composition containing uretdione groups and unsaturated sites. More specifically, the invention pertains to compositions containing multifunctional polyols, uretdiones, free radical curable monomers containing unsaturation and crosslinking agents which may be used to form a high modulus material useful in composites and in the production of prepregs.
Fibers or fabrics pre-impregnated with polymer or resin matrices, “prepregs”, have become increasingly common in the composite industry as they offer several major advantages. One of the biggest advantages is a greater ease of use. Prepregs arrive as solid preformed sheets which can be stored prior to use in composite parts fabrication. Furthermore, methods of composite parts fabrication using hand layup in an open molding process or infusion into closed molds requires the use of liquid resins, solvents and hardeners which can be time consuming, messy and inefficient. For example, in a hand layup process much of the material is wasted in achieving a proper resin mix, and on the rollers, sprayers and other application equipment used to form the composite part.
Moreover, it is difficult to achieve high fiber or fabric content in a composite part using hand layup methods, with an industry standard being about 40% fiber or fabric. The low fiber content and thus increased resin content increases brittleness and diminishes the overall structural properties of the final fabricated composite part. Prepregs, on the other hand, can provide a near perfect resin content which leads to greatly improved strength properties and a higher volume capacity. Prepregs typically contain 65% or more fabric, leading to improved cure times and increased strength. Furthermore, the optimal resin content of prepregs leads to improved uniformity and repeatability of the final composite part. Without the pitfalls of human lamination techniques, there are neither resin rich nor resin dry spots. Thickness is uniform and every part that comes out of the mold has a theoretical likelihood of being identical.
Another advantage of prepregs is that they allow for the use of a wider range of polymer or resin matrices. The accuracy of the machinery used to produce prepregs enables the use of very tough and strong resin systems that would be too high in viscosity to be impregnated by hand.
Today, prepregs find use in commercial aerospace, military/defense, general aviation, space/satellite, marine, sporting goods, automotive, civil engineering, wind energy and the transportation markets. The wind energy industry, for example, is one of the fastest growing consumers of prepregs in the world as energy from wind power is currently the fastest-growing source of electricity production in the world. Contemporary wind turbines are fitted with three blades, each of which can reach 80 meters in length and weigh as much as 35 tons. These blades are typically 70 to 75% E-glass by weight infused with epoxy or unsaturated polyester resins. In 2011, there were 23,640 new wind turbines built globally. At the current growth rate, U.S. wind energy developers install two new wind farms per week, with tens to hundreds of turbines per farm. As such, an area of intensive research and development has been the quality of the rotor blades that are produced, and less expensive production methods and materials.
It is known that prepregs can be produced from a number of different synthetic polymers or resins and various fibers or fabrics. Glass fibers have proved to be a particularly advantageous reinforcing material. Epoxy, vinyl ester, bismaleimide, cyanate ester, phenolic, polyimide, polyetheramide and polyester resins have been processed with glass fibers to create storable prepregs which can be formed by means of heated presses and hardened to generate composite parts combining high strength with rigidity. This process is, however, attended by several disadvantages which often depend on the matrix material selected.
One shortcoming in the abovementioned prepregs is that they have a relatively short shelf-life. Heat cures prepreg materials and storage at warmer temperatures will reduce their shelf-life. Keeping the material cooler, such as by freezing, may extend the shelf-life significantly but adds another set of problems. Prepregs stored at low temperatures need to be wrapped and sealed in polythene and must be fully thawed before breaking the polythene seal in order to avoid moisture contamination. Epoxy resins, for example, are able to absorb water easily, thus storage conditions and processing methods may be critical to obtaining a uniform composite part.
Another shortcoming is that relatively high heats must be applied to the prepreg material in a mold to cure the polymer or resin and form the final composite part. For each prepreg resin system there is a minimum cure temperature and a range of options for cure temperature and duration. The oven or autoclave, the laminate, and all tooling (molds) need to reach and maintain the given cure temperature throughout the specified cure cycle. Frequently, the cure temperature exceeds the temperature that the fiber materials and/or molding equipment can handle. Bismaleimide resins, for example, are cured at 180° C. for three hours during which time the resin exhibits a low viscosity. A typical cure cycle for cyanate ester resins includes temperatures as high as 260° C. Many fibers and fabrics may not withstand such high cure temperatures and the low viscosity makes the molding process problematic.
Additionally, if the resin material has a high exotherm, the heat generated within the prepreg may exceed the temperatures that the fiber materials and/or molding equipment can withstand leading to a discolored or scorched composite part. As such, polymer or resin matrices with large exotherms may only be used in thin layers to avoid excessive heat build-up. Certain epoxy resin matrix formulations, for example, demonstrate exotherms in excess of 300 J/g which can lead to uncontrolled polymerization and excessive heat formation.
An additional shortcoming is the use of solvents, reactive diluents and other toxic materials in certain polymer or resin matrices. For example, unsaturated polyester resins often require reactive diluents such as styrene during the free radical initiated polymerization reaction (cure). Both solvents and reactive diluents have also traditionally been used to lower the viscosity of matrix materials and thus provide for better fiber wet-out properties and improved ease of handling. Reactive diluents are known to lead to a very high crosslink density which can make the final product extremely brittle and reduce its notched impact strength. Solvents, on the other hand, complicate processing as elaborate measures must be taken to extract the solvent vapors and, in many installations, explosion-proof processing equipment becomes necessary.
Alternative polymer matrices include polyurethanes which use uretdiones as the crosslinking agents. Uretidiones are, in effect, self-blocked isocyanates that provide the desired urethane or allophanate linkages while offering a latent heat induced reactivity without the release of toxic blocking agents. The prior art has recognized the advantages associated with this latent reactivity, but has for the most part failed to make practical application of this concept in prepreg matrix formulations which may be cured at lower temperatures.
By way of illustration, disclosures of the preparation of crosslinkable polyurethane rubbers and elastomers which take advantage of the latent reactivity of the uretdione linkage can be found in U.S. Pat. Nos. 3,099,642 and 3,248,370. The processes described in these references involve combining, at temperatures of less than 100° C., a relatively high molecular weight (e.g., on the order of from 500 to 3,000 daltons) difunctional resin, a low molecular weight crosslinking reagent, and a uretdione diisocyanate, or a mixture of a uretdione diisocyanate and a monomeric diisocyanate. The resulting, essentially thermoplastic, formulations contain an excess of isocyanate reactive groups, and these formulations are finally cured to a crosslinked thermoset polymer by treatment at temperatures in excess of 140° C.
U.S. Pat. No. 4,138,372 discloses a prepreg matrix material comprising epoxy resins which are cured using the latent reactivity of the uretdione linkage. Dissociation of the uretdione ring to isocyanates was found to take place at about 170° C. in the absence of catalysts, while catalysts such as tetraphenyl borate-amine complexes lowered the dissociation temperature to some extent. At the high temperatures used in the aforementioned patents, however, the uretdione ring most often dissociates to form free isocyanates.
Furthermore, because of the environmental and economic requirements imposed on the matrix materials, i.e., that they should use as little organic solvent as possible or none at all, for adjusting the viscosity, there is a desire to use raw materials which are already of low viscosity. Known for this purpose are polyisocyanates with an allophanate structure as are described in U.S. Pat. No. 6,392,001.
The formation of allophanate compounds by ring opening of uretdiones with alcohols is known in principle as a crosslinking mechanism in powder coating materials. Nevertheless, the reaction temperatures required for this purpose, in the absence of catalysts, are too high (≧130° C.) for many prepreg applications. Catalysts such as organo-metallic compounds have been found to lower the reaction temperatures (<130° C.; cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419).
Historically, the direct reaction of uretdione rings with alcohols to form allophanates was first investigated for solventborne, isocyanate-free, 2K [2-component] polyurethane coating materials. Without catalysis, this reaction is of no technical importance due to the low reaction rate (F. Schmitt, Angew. Makromol. Chem. (1989), 171, pp. 21-38). With appropriate catalysts, however, the crosslinking reaction between hexamethylene diisocyanate (HDI)-based uretdione curatives and olefinic unsaturated alcohols is reported to begin at <130° C. (U.S. Pat. No. 8,202,618). The use of uretdione groups as curing agents in prepreg materials which cure from 100 to 160° C. has also been disclosed in U.S. Pat. Application 2012/0003890. The prepregs according to the invention use matrix materials which have Tg values of ≧40° C. As such, the matrix materials are dry when applied to the fiber, and the prepreg material lacks any tack. In certain industries, a prepreg material with a certain amount of tackiness may aid in aligning layers in a mold.
It was, therefore, an objective of the present invention to develop new solvent-free, storable matrix compositions which may be hardened at low temperature to form composites which combine high impact strength and dimensional stability. An additional objective of the present invention was to develop new prepreg materials with low exotherms and low pre-cure glass transition temperatures, yet high post-cure glass transition temperatures that would be useful in the renewable energy market.